10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 our $VERSION = '0.30';
16 ## ----------------------------------------------------------------------------
17 ## Setting up our environment ...
18 ## ----------------------------------------------------------------------------
19 ## Class::MOP needs to have a few things in the global perl environment so
20 ## that it can operate effectively. Those things are done here.
21 ## ----------------------------------------------------------------------------
23 # ... nothing yet actually ;)
25 ## ----------------------------------------------------------------------------
27 ## ----------------------------------------------------------------------------
28 ## The code below here is to bootstrap our MOP with itself. This is also
29 ## sometimes called "tying the knot". By doing this, we make it much easier
30 ## to extend the MOP through subclassing and such since now you can use the
31 ## MOP itself to extend itself.
33 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
34 ## ----------------------------------------------------------------------------
36 # We need to add in the meta-attributes here so that
37 # any subclass of Class::MOP::* will be able to
38 # inherit them using &construct_instance
42 Class::MOP::Class->meta->add_attribute(
43 Class::MOP::Attribute->new('$:package' => (
45 # NOTE: we need to do this in order
46 # for the instance meta-object to
47 # not fall into meta-circular death
48 'name' => sub { (shift)->{'$:package'} }
50 init_arg => ':package',
54 Class::MOP::Class->meta->add_attribute(
55 Class::MOP::Attribute->new('%:attributes' => (
57 # NOTE: we need to do this in order
58 # for the instance meta-object to
59 # not fall into meta-circular death
60 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
62 init_arg => ':attributes',
67 Class::MOP::Class->meta->add_attribute(
68 Class::MOP::Attribute->new('$:attribute_metaclass' => (
69 reader => 'attribute_metaclass',
70 init_arg => ':attribute_metaclass',
71 default => 'Class::MOP::Attribute',
75 Class::MOP::Class->meta->add_attribute(
76 Class::MOP::Attribute->new('$:method_metaclass' => (
77 reader => 'method_metaclass',
78 init_arg => ':method_metaclass',
79 default => 'Class::MOP::Method',
83 Class::MOP::Class->meta->add_attribute(
84 Class::MOP::Attribute->new('$:instance_metaclass' => (
86 # NOTE: we need to do this in order
87 # for the instance meta-object to
88 # not fall into meta-circular death
89 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
91 init_arg => ':instance_metaclass',
92 default => 'Class::MOP::Instance',
96 ## Class::MOP::Attribute
98 Class::MOP::Attribute->meta->add_attribute(
99 Class::MOP::Attribute->new('name' => (
101 # NOTE: we need to do this in order
102 # for the instance meta-object to
103 # not fall into meta-circular death
104 'name' => sub { (shift)->{name} }
109 Class::MOP::Attribute->meta->add_attribute(
110 Class::MOP::Attribute->new('associated_class' => (
112 # NOTE: we need to do this in order
113 # for the instance meta-object to
114 # not fall into meta-circular death
115 'associated_class' => sub { (shift)->{associated_class} }
120 Class::MOP::Attribute->meta->add_attribute(
121 Class::MOP::Attribute->new('accessor' => (
122 reader => 'accessor',
123 predicate => 'has_accessor',
127 Class::MOP::Attribute->meta->add_attribute(
128 Class::MOP::Attribute->new('reader' => (
130 predicate => 'has_reader',
134 Class::MOP::Attribute->meta->add_attribute(
135 Class::MOP::Attribute->new('writer' => (
137 predicate => 'has_writer',
141 Class::MOP::Attribute->meta->add_attribute(
142 Class::MOP::Attribute->new('predicate' => (
143 reader => 'predicate',
144 predicate => 'has_predicate',
148 Class::MOP::Attribute->meta->add_attribute(
149 Class::MOP::Attribute->new('init_arg' => (
150 reader => 'init_arg',
151 predicate => 'has_init_arg',
155 Class::MOP::Attribute->meta->add_attribute(
156 Class::MOP::Attribute->new('default' => (
157 # default has a custom 'reader' method ...
158 predicate => 'has_default',
163 # NOTE: (meta-circularity)
164 # This should be one of the last things done
165 # it will "tie the knot" with Class::MOP::Attribute
166 # so that it uses the attributes meta-objects
167 # to construct itself.
168 Class::MOP::Attribute->meta->add_method('new' => sub {
173 (defined $name && $name)
174 || confess "You must provide a name for the attribute";
175 $options{init_arg} = $name
176 if not exists $options{init_arg};
178 # return the new object
179 $class->meta->new_object(name => $name, %options);
182 Class::MOP::Attribute->meta->add_method('clone' => sub {
184 $self->meta->clone_object($self, @_);
195 Class::MOP - A Meta Object Protocol for Perl 5
199 # ... This will come later, for now see
200 # the other SYNOPSIS for more information
204 This module is an attempt to create a meta object protocol for the
205 Perl 5 object system. It makes no attempt to change the behavior or
206 characteristics of the Perl 5 object system, only to create a
207 protocol for its manipulation and introspection.
209 That said, it does attempt to create the tools for building a rich
210 set of extensions to the Perl 5 object system. Every attempt has been
211 made for these tools to keep to the spirit of the Perl 5 object
212 system that we all know and love.
214 =head2 What is a Meta Object Protocol?
216 A meta object protocol is an API to an object system.
218 To be more specific, it is a set of abstractions of the components of
219 an object system (typically things like; classes, object, methods,
220 object attributes, etc.). These abstractions can then be used to both
221 inspect and manipulate the object system which they describe.
223 It can be said that there are two MOPs for any object system; the
224 implicit MOP, and the explicit MOP. The implicit MOP handles things
225 like method dispatch or inheritance, which happen automatically as
226 part of how the object system works. The explicit MOP typically
227 handles the introspection/reflection features of the object system.
228 All object systems have implicit MOPs, without one, they would not
229 work. Explict MOPs however as less common, and depending on the
230 language can vary from restrictive (Reflection in Java or C#) to
231 wide open (CLOS is a perfect example).
233 =head2 Yet Another Class Builder!! Why?
235 This is B<not> a class builder so much as it is a I<class builder
236 B<builder>>. My intent is that an end user does not use this module
237 directly, but instead this module is used by module authors to
238 build extensions and features onto the Perl 5 object system.
240 =head2 Who is this module for?
242 This module is specifically for anyone who has ever created or
243 wanted to create a module for the Class:: namespace. The tools which
244 this module will provide will hopefully make it easier to do more
245 complex things with Perl 5 classes by removing such barriers as
246 the need to hack the symbol tables, or understand the fine details
249 =head2 What changes do I have to make to use this module?
251 This module was designed to be as unintrusive as possible. Many of
252 its features are accessible without B<any> change to your existsing
253 code at all. It is meant to be a compliment to your existing code and
254 not an intrusion on your code base. Unlike many other B<Class::>
255 modules, this module B<does not> require you subclass it, or even that
256 you C<use> it in within your module's package.
258 The only features which requires additions to your code are the
259 attribute handling and instance construction features, and these are
260 both completely optional features. The only reason for this is because
261 Perl 5's object system does not actually have these features built
262 in. More information about this feature can be found below.
264 =head2 A Note about Performance?
266 It is a common misconception that explict MOPs are performance drains.
267 But this is not a universal truth at all, it is an side-effect of
268 specific implementations. For instance, using Java reflection is much
269 slower because the JVM cannot take advantage of any compiler
270 optimizations, and the JVM has to deal with much more runtime type
271 information as well. Reflection in C# is marginally better as it was
272 designed into the language and runtime (the CLR). In contrast, CLOS
273 (the Common Lisp Object System) was built to support an explicit MOP,
274 and so performance is tuned for it.
276 This library in particular does it's absolute best to avoid putting
277 B<any> drain at all upon your code's performance. In fact, by itself
278 it does nothing to affect your existing code. So you only pay for
279 what you actually use.
281 =head2 About Metaclass compatibility
283 This module makes sure that all metaclasses created are both upwards
284 and downwards compatible. The topic of metaclass compatibility is
285 highly esoteric and is something only encountered when doing deep and
286 involved metaclass hacking. There are two basic kinds of metaclass
287 incompatibility; upwards and downwards.
289 Upwards metaclass compatibility means that the metaclass of a
290 given class is either the same as (or a subclass of) all of the
293 Downward metaclass compatibility means that the metaclasses of a
294 given class's anscestors are all either the same as (or a subclass
297 Here is a diagram showing a set of two classes (C<A> and C<B>) and
298 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
299 metaclass compatibility both upwards and downwards.
301 +---------+ +---------+
302 | Meta::A |<----| Meta::B | <....... (instance of )
303 +---------+ +---------+ <------- (inherits from)
306 +---------+ +---------+
308 +---------+ +---------+
310 As I said this is a highly esoteric topic and one you will only run
311 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
312 are interested in why this is an issue see the paper
313 I<Uniform and safe metaclass composition> linked to in the
314 L<SEE ALSO> section of this document.
316 =head2 Using custom metaclasses
318 Always use the metaclass pragma when using a custom metaclass, this
319 will ensure the proper initialization order and not accidentely
320 create an incorrect type of metaclass for you. This is a very rare
321 problem, and one which can only occur if you are doing deep metaclass
322 programming. So in other words, don't worry about it.
326 The protocol is divided into 3 main sub-protocols:
330 =item The Class protocol
332 This provides a means of manipulating and introspecting a Perl 5
333 class. It handles all of symbol table hacking for you, and provides
334 a rich set of methods that go beyond simple package introspection.
336 See L<Class::MOP::Class> for more details.
338 =item The Attribute protocol
340 This provides a consistent represenation for an attribute of a
341 Perl 5 class. Since there are so many ways to create and handle
342 atttributes in Perl 5 OO, this attempts to provide as much of a
343 unified approach as possible, while giving the freedom and
344 flexibility to subclass for specialization.
346 See L<Class::MOP::Attribute> for more details.
348 =item The Method protocol
350 This provides a means of manipulating and introspecting methods in
351 the Perl 5 object system. As with attributes, there are many ways to
352 approach this topic, so we try to keep it pretty basic, while still
353 making it possible to extend the system in many ways.
355 See L<Class::MOP::Method> for more details.
363 There are very few books out on Meta Object Protocols and Metaclasses
364 because it is such an esoteric topic. The following books are really
365 the only ones I have found. If you know of any more, B<I<please>>
366 email me and let me know, I would love to hear about them.
370 =item "The Art of the Meta Object Protocol"
372 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
374 =item "Putting MetaClasses to Work"
376 =item "Smalltalk: The Language"
384 =item Uniform and safe metaclass composition
386 An excellent paper by the people who brought us the original Traits paper.
387 This paper is on how Traits can be used to do safe metaclass composition,
388 and offers an excellent introduction section which delves into the topic of
389 metaclass compatibility.
391 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
393 =item Safe Metaclass Programming
395 This paper seems to precede the above paper, and propose a mix-in based
396 approach as opposed to the Traits based approach. Both papers have similar
397 information on the metaclass compatibility problem space.
399 L<http://citeseer.ist.psu.edu/37617.html>
407 =item The Perl 6 MetaModel work in the Pugs project
411 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
413 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
419 =head1 SIMILAR MODULES
421 As I have said above, this module is a class-builder-builder, so it is
422 not the same thing as modules like L<Class::Accessor> and
423 L<Class::MethodMaker>. That being said there are very few modules on CPAN
424 with similar goals to this module. The one I have found which is most
425 like this module is L<Class::Meta>, although it's philosophy and the MOP it
426 creates are very different from this modules.
430 All complex software has bugs lurking in it, and this module is no
431 exception. If you find a bug please either email me, or add the bug
436 I use L<Devel::Cover> to test the code coverage of my tests, below is the
437 L<Devel::Cover> report on this module's test suite.
439 ---------------------------- ------ ------ ------ ------ ------ ------ ------
440 File stmt bran cond sub pod time total
441 ---------------------------- ------ ------ ------ ------ ------ ------ ------
442 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 9.6 100.0
443 Class/MOP/Attribute.pm 100.0 100.0 91.7 73.8 100.0 28.4 92.1
444 Class/MOP/Class.pm 100.0 93.5 82.3 98.2 100.0 56.6 95.7
445 Class/MOP/Method.pm 100.0 64.3 52.9 80.0 100.0 3.5 85.3
446 metaclass.pm 100.0 100.0 80.0 100.0 n/a 1.9 97.4
447 ---------------------------- ------ ------ ------ ------ ------ ------ ------
448 Total 100.0 90.8 79.7 86.2 100.0 100.0 93.6
449 ---------------------------- ------ ------ ------ ------ ------ ------ ------
451 =head1 ACKNOWLEDGEMENTS
455 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
457 Thanks to Rob for actually getting the development of this module kick-started.
463 Stevan Little E<lt>stevan@iinteractive.comE<gt>
465 =head1 COPYRIGHT AND LICENSE
467 Copyright 2006 by Infinity Interactive, Inc.
469 L<http://www.iinteractive.com>
471 This library is free software; you can redistribute it and/or modify
472 it under the same terms as Perl itself.